Implement set in pure functional way

pure_set.js

const empty = () => y => window.Error("not found error");
const extendSet = (y, val, set) => x => x === y ? val : set(x);

const setA = extendSet('a', 3, empty());
setA('a'); // 3
setA('b'); // not fuond error

const setA2 = extendSet('b', 4, setA);
setA2('a'); // 3
setA2('b'); // 4

const setA3 = extendSet('a', 42, setA2);
setA3('a'); // 42
setA3('b'); // 4